A good resistance to self-ignition, expressed as high octane number, is necessary in the gasoline engines in order to obtain a good engine efficiency and therefore a higher fuel economy and a low pollution level.
Up to few years ago such goals were, and partly still today are, reached by adding an antiknock to the gasoline, essentially tetraethyl- and/or tetramethyl-lead. Nevertheless the traditional engines exhaust gases contain highly harmful substances, such as carbon monoxide, hydrocarbons, nitrogen oxides, sulphur oxides and lead, the latter being dangerous because of the effects that it may have on the central nervous system and because of the strong carcinogenic potential of further added substances (called "scavengers"), necessary to remove lead from the engine chamber deposits and transformed into dioxins during the combustion. Therefore it has been decided, starting from the United States of America, to abate the pollutants by promoting their conversion into harmless, or at least much less dangerous gases, by means of catalytic converters set on the engine exhaust outlet, reducing in this way the emitted quantity of them. The lead, however, rapidly inactivates the converter catalyst, and also for this reason, besides to avoid the above recalled health hazards, the decision had to be taken to remove it from the gasolines. To obtain this, it has been agreed to start the exclusive commercialization of vehicles equipped with catalytic converter, and hence, parallelly, to commercialize unleaded gasolines, and to progressively reduce the production and the sales of leaded gasolines, as well as the gasoline lead content, in parallel with the reduction of the circulating cars that use them. But in order to maintain the engines efficiency it is necessary to keep a high octane number, which was obtained by increasing the benzene and other aromatic compounds content. The benzene is a highly toxic compound, the most toxic molecule of gasoline, and it is recognized as one of the most powerful carcinogenic agents. With the increase of the gasoline benzene content the benzene emissions to the air rapidly increase, both from evaporation due to its high volatility, and from after-combustion combustion. For these reasons its concentration in the gasolines results by far the most critical parameter from the health hazard point of view.
The content of benzene and of the other aromatics in the unleaded gasolines was, at the beginning of their commercialization, respectively 5% and 60% maximum. Subsequently, the different national legislations considered abatements of this component; in the USA, the only country in the world prescribing such a low limit, starting from 1.1.95 the benzene content is fixed at 1% volume maximum, for the reformulated gasolines, whose market share is about 25%.
In the European Union, starting from October the 1st, 1989, the allowed level is at maximum 5% volume, with a forecast of gradually lowering it in the future. In Italy, since the beginning of 1993, the law prescribed a benzene content not higher than 3%, meant as the quarterly weighted average of the sales coming from each refinery. Recently, as a spontaneous initiative, a national company put on sale on the whole national territory unleaded gasoline with a lower than 1% benzene content.
Generally speaking, a petroleum refinery, producing several products among which gasoline, comprises an atmospheric distillation unit that is fed by the crude oil and produces a light fraction composed by gas and LPG, an intermediate fraction called "virgin naphtha" and heavier fractions. The virgin naphtha feeds a splitting unit, giving rise to a fraction, called "light tops", essentially composed of five carbon atoms hydrocarbons (C5) (and in certain units also of the lower-boiling part of the six carbon atoms molecules), and to a heavy fraction, called heavy naphtha, containing the C6 molecules (or in the above units the higher-boiling part of C6) and other molecules, up to C11. The cutting temperature is 65-70.degree. C. The light tops are usually fed to an isomerization processing unit, that produces gas and LPG, as light fraction, and isomerization gasoline, or isomerate, to be sent to the finished gasoline blending.
The heavy naphtha is fed to a catalytic reforming processing unit, from which gas and LPG again are obtained from one side and reforming gasolines, to be sent to the finished gasoline blending, from the other side.
It is just the C6 higher-boiling fraction, containing the benzene precursors, that in the reformer produces a considerable amount of benzene and consequently a noteworthy pollution from evaporation, already in the refinery.
Therefore the production of benzene is a physiological fact of the oil refining, but till now it has not been considered economically possible to limit it.
In Oil & Gas Journal, April the 27th, 1987, page 74, a meeting among representatives of the big worldwide oil companies is reported, in which a crucial point has been the way to abate in the refinery the benzene content; among the discussed methods the simplest one is to extract the benzene produced in the reformer. This method adds up an operation, with the relevant investment and operating costs, to the production process, keeps the problem of benzene pollution around the refinery and presents on top of that the one of the cumulated benzene disposal.
Other methods have been reminded, among which in particular the shifting of the reformer feed cutting point towards higher initial boiling temperatures, that is towards the exclusion of at least a fraction of the high-boiling C6. It is anyway affirmed that this method is antieconomic.
It is also clearly said that the removal of the benzene precursors from the reformer feed doesn't eliminate the problem, because there are also other benzene sources, such as the dealkylation of higher carbons number aromatics. The same problem is faced up by the CONCAWE (defined "the oil companies' european organization for environmental and health protection") in a publication entitled "Economic consequences of limiting benzene/aromatics in gasoline". The Hague, July 1989, page 10. Here in particular it is asserted that the most effective method to reduce the reformate benzene would be to increase the initial boiling point of the reformer feed, but this method would carry some problems, particularly the lower reformer throughput, the fact of having a heavier feedstock with operating problems and the decrease of the product octane number. The solution for this last drawback could be that of dividing the "straight run" gasoline into a fraction, C5+light C6, to be isomerized, and in a heavier fraction to be directly sent to the gasoline blending. This method is considered expensive. For the benzene reduction costs minimization this publication indicates in an unequivocal way a cutting temperature of 66.degree. C., implying to send all the benzene precursors to the catalytic reformer.
In the summary of this publication it is declared that "Further reduction of benzene below 3% vol would need installation of additional isomerization and benzene extraction facilities also in complex refineries". A benzene limit at 1% volume would require an investment of 1750 millions dollars for the European Community refineries. The manufacturing cost would increase by 16-20 dollars per ton for the simple refineries and by 8-12 dollars per ton for the complex refineries. And, moreover, at the 3.1.3 paragraph, it is admitted that the benzene target of 1% volume can be reached only by extracting it from the reformate, and this for all the examined process schemes.
In a more recent CONCAWE publication ("Catalogue of CONCAWE special interest reports", Brussels, January 1996, page 31) it is still reminded that decrease of the benzene level in the unleaded gasolines beneath 1% would require a global investment of around 1750 million dollars only in Europe, would considerably increase the costs of production and in any case it would let the problem of eliminating two millions tons per year of extracted benzene unsolved. Similar concepts to the previously expressed ones can be found at page 23 of the report "Gasoline processing for the 1990's" of the UOP 1990 Fuels Technology Conference, held in Montecarlo. Here as well the method of removing the benzene precursors from the reformer feed and of charging them to the isomerization is considered interesting but little feasible, because of the increased hydrogen consumption for the saturation of benzene and of its precursors, of the difficulty to remove a sufficient quantity of precursors in certain types of feedstocks, of the benzene production by superior aromatics dealkylation in the reformer and overall because of the octane loss that would be implied by charging the precursors containing stream itself to the isomerization instead than to the catalytic reformer.
Similar conclusions to the above mentioned ones are contained as well in "Dossier Benzene",--published March the 20th, 1995 by the Unione Petrolifera, an organization gathering all the Italian oil companies. In this report all the possible refinery interventions are listed, that is:
the reduction/elimination of the precursors;
the reduction of the reformer severity;
the extraction and the treatment of benzene.
The first alternative would assume, according to the report, the possibility of the crude oils selection (limiting the operating flexibility and hence antieconomonic) and the change of the feed distillation range; in this respect the light tops cutting point increase to 95.degree. C. would allow to remove almost all the benzene precursors but it would generate a series of collateral problems, such as the throughput reduction and the unoptimization of the catalytic reforming process, and the increase of the straight-run fraction share in the gasolines pool.
Other methods examined in the report "that do not act directly on the benzene or on its precursors are in connection with the blending in the gasolines pool of components coming from isomerization and alkylation units", that, as the report follows, allows a reformer severity reduction and a dilution of the benzene containing fraction; however these methods have little effectiveness in order to solve the problem in consideration of the obvious limitations of the possible use of these components.
However in any case the problem remains of reaching a satisfactory octane number, for instance by additional bending of antiknocks, such as the MTBE (methyl-ter-butyl-ether), or by increasing the reformer operating severity lowering the yield, both expensive solutions.
It is evident that, so far, the solutions considered feasible of the gasolines benzene content limitation problem, particularly beneath 1% volume, are all expensive and not lacking additional complex problems, such as the use of a benzene production surplus correspond to about 40% of the present market, already practically saturated (See Concawe and U.P. reports).